Accelerometers find use in widely diverse applications including automobile air bags and suspension systems, computer hard disc drivers, smart detonation systems for bombs and missiles and machine vibration monitors. Silicon micromachined acceleration sensors are beginning to replace mechanical acceleration switches. Present accelerometers are all based upon the classical Newtonian relationship of force, F, mass, m, and acceleration, a, in which F=ma. Thus, for a cantilevered beam, the force due to acceleration causes the beam to deflect. This deflection is sensed either by sensing the change in piezo resistors or by a change in capacitance. Such systems are not stable over wide temperature ranges and have a response which peaks due to insufficient mechanical damping.
One form of accelerometer made by bulk micromachining consists of a membrane or diaphragm of silicon formed by chemical etching having a large mass of silicon at the centre and tethers of thin film piezo-resistors, whose resistance is sensitive to strain and deformation, suspending the mass. Vertical acceleration causes the large silicon mass to move, deforming the diaphragm and changing the resistance of the piezo-resistors. Such bulk micromachined devices are large by integrated circuit standards and consistent with semiconductor circuit fabrication techniques, requiring the signal conditioning to be off-chip.
Another system made by surface micromachining, is based on a differential capacitor. Surface micromachining creates much smaller, more intricate and precisely patterned structures than bulk micromachining. It involves the same process that is used to make integrated circuits, namely, depositing and etching multiple thin films and layers of silicon and silicon-oxide to form complex mechanical structures. In this case a central beam is affixed in an "H" configuration with the spaced apart parallel arms of the "H" supporting respective ends of the cross beam.
A plate affixed perpendicular to the beam forms a moving capacitor plate that is positioned between two fixed plates, thus, forming two capacitors sharing a common moving plate. When the unit is subjected to an accelerating force the beam and hence moving plate moves closer to one of the fixed plates and away from the other fixed plate. The effect is to reduce one of the capacitors and increase the other by an amount proportional to the acceleration. The device requires proper orientation with the cross beam parallel to the direction of acceleration. However, surface micromachining is used to create a much smaller device adapted to the same techniques used to make integrated circuits. The moving capacitor plate accelerometer suffers from high noise at low frequencies and exhibits drift at low acceleration measurements.
It is an object of the present invention to provide an improved accelerometer. It is a further object of the invention to provide an accelerometer having no proof mass and a corresponding increase ruggedness.